1. Field of the Invention
This invention relates to a method of making a semiconductor device and particularly to improvements in a method for forming a conductive region for applicable to a transistor in a bipolar type integrated circuit devices.
2. Discussion of Background
A typical bipolar transistor comprises a semiconductor substrate having a 1st conductive type (e.g., n-type) epitaxial semiconductor layer at the surface side thereof, in which an n-type region is isolated by an p-n junction or an insulating region to constitute a collector region. A base region is a 2nd conductive type (e.g., p-type) doped region formed in the collector region at the surface of the epitaxial layer by the ion implantation and solid-phase diffusion using the 2nd conductive type impurity, and an emitter region of an n-type doped region is formed in the base region by the ion implantation and solid-phase diffusion using the 1st conductive type impurity.
In this typical bipolar transistor, a width of the 2nd conductive type semiconductor layer (i.e., base) must have a considerably narrower region without greatly increasing the base sheet resistance, in order to greatly improve the properties of the device. However, it is difficult to control and to be narrow the base width by using the ion diffusion etc. as above described conventional method, and it is also very difficult to make a sharp change in the impurity concentration distribution at the p-n junction which is a boundary surface of the 1st conductive type semiconductor layer and the 2nd conductive type semiconductor layer.